Long-term characterization and resource potential evaluation of the digestate from food waste anaerobic digestion plants
Graphical abstract
Introduction
Anaerobic digestion (AD) is one of the most promising methods for bioenergy conversion from organic waste, such as food waste (FW), sewage sludge, and manure (Appels et al., 2011; Xu et al., 2018). It has been reported that approximately 1.3 billion tonnes of FW are produced every year, which is one-third of food production (Morone et al., 2019). China has the most production of FW in the world (Jin et al., 2021), over 100 million tonnes of FW were produced every year, and most of the FW was treated by AD. As a result, a considerable quantity of digestate from FW (DFW) was generated, which needs to be appropriately treated (C. Yang et al., 2020; Z. Yang et al., 2020; Liu et al., 2020). The treatment methods of digestate include as a soil amendment to remove polycyclic aromatic hydrocarbons and total petroleum hydrocarbons from the petroleum-contaminated soils (Bianco et al., 2020; Gielnik et al., 2021), landfilling, and incineration (Peng et al., 2019; Chen et al., 2021). However, these disposal methods would result in secondary contamination, such as leachate and air pollution. Therefore, DFW management is becoming a significant bottleneck for food waste AD due to its large quantity production and potential environmental pollution.
It has been reported that DFW retains a considerable amount of organic matter (OM) (Liu et al., 2020; Koido et al., 2018). The protein and lignin contents of DFW are even higher than FW due to the low biodegradability of the lignin and protein and limited hydraulic retention time (HRT) during AD (Tampio et al., 2015). The commonly used HRT in a full-scale anaerobic digester is 15–20 days, related to the type of feedstock (Anyaoku and Baroutian, 2018; Srisowmeya et al., 2020). These organic matters could be transformed into biochar, bio-oil, and syngas by pyrolysis technology (Hung et al., 2017). In addition, DFW also has abundant nutrient elements, such as nitrogen (N), phosphorus (P), and potassium (K) (Opatokun et al., 2017; Grigatti et al., 2020). These nutrients of DFW are necessary for plant growth and could be recovered by composting as fertilizer or soil amendment (Cristina et al., 2020). Among these nutrients, nitrogen is the most critical nutrient of digestate for its high amount (Czekała et al., 2020). Due to the difference of nitrogen speciation (i.e., ammonia nitrogen and organic nitrogen) in digestates from different AD feedstocks, the dose of digestate is dependent on the nutrient's content and the plant requirement (Cristina et al., 2020). Several national legislations categorically exclude the application of a certain quantity of digestate per hectare, such as in European Union countries (Drosg et al., 2015). Therefore, understanding the speciation and concentration of nutrients was very important for the digestate as the fertilizer.
To beneficial use DFW, it is crucial to understand the characteristics of DFW fully. Some studies so far have focused mainly on the characteristics of DFW collected from lab-scale digesters (Tampio et al., 2015, Tampio et al., 2016). It is well known that the scale size (laboratory-scale or industrial-scale) of the anaerobic digester influences DFW characteristics, such as OM content and element composition (Liu et al., 2020; Tampio et al., 2015). A few studies reported DFW characteristics from industrial-scale digesters, with only selected characteristics based on instantaneous or snapshot measurements (Liu et al., 2020; H. Chen et al., 2019). The detailed and long-term characteristics of DFW from industrial-scale food waste AD plants were rarely reported, which significantly limits the utilization of DFW.
This study monitored the detailed and long-term characteristics of the DFW from an industrial-scale food waste AD plant for 16 months. The objective was to fully understand the characteristics of the DFW, including the physicochemical characteristics, biochemical methane potential (BMP), nutrient content and speciation, metal concentration and speciation, and their seasonal variations. Based on the long-term characteristics of the DFW, pyrolysis and composting were investigated as potential resource utilization methods. The research findings would provide a better understanding of DFW's characteristics to choose an appropriate method for resource recovery.
Section snippets
Sampling of food waste digestate
The digestate samples were collected from a typical industrial-scale food-waste AD plant located in Shenzhen, China. The plant adopts a two-phase AD with a treatment capacity of 300 tons per day, and it has been operated successfully for over five years. The collected FW firstly undergoes a pretreatment process to remove the impurities and to recycle oil. After the pretreatment, the FW slurry is pumped into two reactors (the total reaction volume is 800 m3) for acidogenesis, and the hydraulic
Basic characteristics of the DFW
The basic characteristics of the DFW are shown in Fig. 1, including the TS, VS, conductivity, pH, elemental composition, and HHV. The values of these parameters changed within narrow ranges during the 16 months test period. The monthly TS was higher than 40% for most of the time, with an average annual value of 41.73% (Fig. 1a). In China, the moisture content of the landfilled digestate is required less than 60% (GB 16889-2008). The results indicated that the tested DFW had met the national
Conclusions
The DFW from an industrial-scale food-waste AD plant presented a relatively stable behavior. The DFW contained considerable amounts of biodegradable OM (23–40% for lignin and 12–26% for protein) and abundant nutrients (N, P, and K), indicating that the DFW had considerable potential for resource utilization. Compared to other digestate substrates, the concentrations of metals such as Ca (55.17 mg g−1) and Fe (15.55 mg g−1) were higher for DFW.
Based on the characterization of DFW, pyrolysis and
CRediT authorship contribution statement
Ning Wang: Draft preparation, Methodology, Conduct Experiments, Experiment design. Dandan Huang: Writing - review & editing. Chao Zhang: Writing - review & editing. Mingshuai Shao: Conduct Experiments. Qindong Chen: Data curation. Jianguo Liu: Funding acquisition and Methodology. Zhou Deng: Resources. Qiyong Xu: Supervision, Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the National Key Research and Development Program of China (2018YFC1902903) and Shenzhen Science and Technology Innovation Committee (JCYJ20190806145607372).
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